This is the tube which as a rule is preferred by Audio Research and Conrad Johnson for inclusion in their creations. Clearly, both manufacturers have a different approach and a decades-old background, yet to this day they begin and build on their sound using this tube, which they consider to be dependable and reasonably priced. It didn’t take us long to realise that, wherever we used the SED 6550C, we were dealing with a very balanced output, possessing a considerably adequate and extended low bass. A calm, warm, gentle audio contribution in every case, with smooth, detailed highs, without any aggressive characteristics.Concerning image and scenic dimensions, it got the green light both for the familiar American push-pulls, as well as more broadly, wherever else it was called upon to perform. Add to this a musical, open midrange with notable frequency extension but without particularly loose lows. [Production date: 1/06]

Black Sable SED 6550C Cryogen

Black Sable, fastidiously measuring the SED 6550C, selects the steadiest and best in electrical performance and manufacture and runs them through a cryogenesis process. This fastidiousness and methodology raises positive results to the surface (a fact that affects also the sound, I always found new sound characteristics on cryogenetic treated cables or -of course- guitar strings!) : an even better image, purer timbres, richer 'color' palette, midrange liquidity and ever-so-slightly better articulated lows - all of them in small doses. If we were asked whether it is worth the additional cost, we can easily answer yes. To this should be added an even more complete harmonic substance, which is clearly attributable to the cryogenesis process. If nothing else, we start and end this comment with the fact that they are more dependable as a long-term solution, since they “go in depth” further, while transferring the musical message. They require about 30 hours of burn-in. [Year of purchase: 2009]

TungSol 6550

Today’s Tungsol 6550 performed, in my opinion, better than all present day 6550s in home amplifiers. With slightly broader extension and richer lows than the SEDs, an even larger soundstage in both height and depth, with excellent detail in the entire mid-range and an extended frequency spectrum, it won me over from the first moment, making me listen again and again to familiar recordings of both classical and jazz, as well as rock music, for hours on end. It is a genuinely excellent 6550 as far as sound is concerned; combining a lush mid-range with an excellent focus on performers, an low frequencies with volume and … confidence, without being plagued by problems of emphasis in the mid-low region, or of verbosity. The bass “blends in” temporally and is rendered with consistency, rhythm, and in harmony with the rest of the music. It was one of the warmest sounds of the group, without overdoing it by blurring the image or lacking in harmonics – in fact the harmonic range and presence are excellent, while the image is in no way inferior to the other 6550s in the group. The feeling of precision of everyone participating in the performance, together with a fine, yet at the same time detailed, presentation of the instruments’ texture, classifies it amongst the top-ranking opportunities. It showed great preference in Conrad Johnson’s power amplifiers, as well as in Primaluna’s integrated and power amps. Finally, slightly clearer dynamic contrasts should be added to the above, compared to the 6550C. It requires at least 50 hours of burn-in. [Production date: 5/08]

Electro Harmonix KT88

This is the tube with the leanest high's and low's. Image and precision in the foreground, with a finer, fragile and ethereal presence, contribute to the rendering of vocals with an abundant feeling of fluidity and finesse. It definitely does not possess Tungsol 6550’s volume of lows and the extremely warm mid-range, but this is counterbalanced by an improvement in resolution as well as better information concerning the dimensions of each recording room, thus making whatever reverberations and reflections of its space more clearly discernible. In addition, it offers more detailed low frequencies and faster dynamics. It is intrinsically brighter in relation to all the 6550s of our trial; however, we noted that this additional resolution has its drawback at the ends of the range, with the cymbals, for example, of familiar recordings being rendered slightly more steely and brightly compared to the other tubes. At the same time, of course, the more precise location of performers in space is impressive. The highs gain in expanse and are freed to the extent where they might be characterized revelatory in some cases. In our amplifiers we did not notice any additional grain or harshness caused by use – quite the opposite in fact. One hundred or more hours of burn-in are required before they reach maximum performance. [Production date: 2/08]

Electro Harmonix 6550

It offers a “fuller” mid range than the EH-KT88, with brighter and sharper highs than the SED and Tungsol, and even the EH. Some “verbosity” is noticeable in the mid-lows, which makes the left notes on the piano sound expanded, but somewhat dry and blurred. I’d rather use the word “veil” in this case, since in some classical piano recordings, as well as rock, the low octaves blurred even further. If you consider that most (maybe more classic and older) push-pull amplifiers are ruled by such a character, which expresses itself in all its glory during reproductions of piano recordings, I would never advise the “blind” purchase of a set of four or eight of these without trial. [Production date 2005]

JJ KT88

From the first moment they impress with their physical weight and their external quality of manufacture. During the first few hours the output is rather fine, dry and lacking slightly in focus, however after the first hundred hours, they demonstrate an excellently vigorous and fast low, not as extensive as that of the Tungsol. They are gentle over the entire range, with a slightly dry and warm mid-range full of clarity, thus contributing towards establishing a scenic surrounding which is less energetic than the Electro Harmonix KT88 and KT90. The macro-dynamic contrasts move within the average, as well as the micro-dynamic ones. The taught bass will be greatly appreciated by those who listen to jazz recordings more frequently, particularly recordings with acoustic bass. Generally speaking, we believe that in all amplifiers, they offered sufficient resolution, low noise and greatly contributed towards our spending with them a restful, lengthy listening, switching from one musical genre to the other. They have a good quality of manufacture, although opinions differ at times – some consider them to be amongst the most reliable, while others have been troubled by problems and noises. For years now, in our hands, they have presented no problems. [Years of production 2006 & 2002]

Genalex Gold Lion KT88 Reissue

To begin with, it is with great pleasure that we see the revival of this great name. We then note that the tube is characterised by top-quality manufacture, so without further ado we move on to the listening. (These tubes had done 50 hours in special measuring and matching equipment, as well as a few more hours of testing.) From the start one is impressed by the magic which it restores to sound, a musical aura with detail, presence and charm that characterises the top NOS (new old stock) tubes. Following that, the feelings raised bring to mind the exaggerated phrase: “Present-day push-pulls with Gold Lion's tetrodes sound very close to triode standards, obviously with the advantages of surplus power”. Its head designer Cary Dennis Had, stated that during the design phase of the company’s new top KT88 push-pull amplifiers, he used four old NOS Genalex Gold Lion, that he had to hand, in one channel and an equal number of new circulation ones in the other. The difference was non existent so he decided, in high spirits, to use them to equip his reference amplifiers. It is obvious that his intention is to bridge the gap between the reference sound of the past and today’s, to traverse the time gap using the best possible vehicle up to now. Vocal works, operas, jazz, even powerful rock productions keep you rooted, and forcing you to revise the performance of the amplifiers you posses. It requires about 80-100 hours of burn-in before the highs tie in tightly with the rest of the frequency range, so that even the finest details are brought to the surface in a richer and more natural manner than ever before. [Year of production 2008]

Electroharmonic KT90

This is the tube preferred by the legendary E.A.R.-Yoshino, Jadis (Ei KT90) and Manley as the one that was to carry the voltage and underline the sound of their top creations. They follow a number of recent creations which include it. I do not hesitate to declare that in many applications (being compatible in place of the KT88s); I liked this tube sufficiently, even a lot. It has its keen fans but there are also several people who simply are indifferent to its sound. Everywhere we observed a rich, particularly rhythmic tight low, a broadening of the width of scenic horizon and, finally, more than adequate information concerning the recording spaces and the reverberation of sound within them. It aided the amplifiers to express themselves with greater ease, effecting a slightly spot-lit mid range.With the KT90 voices are presented as warm – yet not dry – and crystal-clear. The musicians reveal an imposing presence. Some discern a more mechanical articulation in the mid-range but we simply observed that its acoustic character does not fit everywhere, or always. This is the only one still produced (also in a cryogen edition which however we have not tested), since the plant of Ei has for the time being stopped its production and suffered damages. [Year of production 2006]